1. Field of the Invention
The present invention is related to an apparatus, method, and system for adjusting a vertical regist shift between lines, pages or colors in a multiple beam recording system, and more particularly is directed to controlling the writing of information from the multiple beams to limit the vertical regist shift.
2. Discussion of the Background Art
A multiple beam optical scanning system for writing information onto a photoconductive drum is shown in FIG. 1. In this multiple beam optical system, two light sources 25 and 26 are provided for generating laser beams for effecting writing onto the photoconductive drum 14. The laser beams output of the light sources 25 and 26 pass through a collimator lens 27, reflect off a polygonal mirror 28, pass through a further lens 29, and then impinge on the photoconductive drum 14. The polygonal mirror 28 is rotated by a motor 30 and the photoconductive drum 14 is rotated by a motor 32. Further, a photodetector 31 is provided for detecting a synchronous signal to effect scanning across the photoconductive drum 14 in the scanning direction. With this structure, the light sources 25 and 26 generate laser beams which scan across the photoconductive drum 14 in a scanning direction, and which generate different lines of information in a sub-scanning direction (i.e. the direction of rotation of the photoconductive drum 14).
This system shown in FIG. 1, however, may suffer from a problem of a vertical regist shift during the following situation. The detected signal from the photodetector 31 is utilized as a synchronizing signal for the scanning direction. An image forming start signal for the sub-scanning direction, to start rotation of the photoconductive of drum 14, is also generated. Further, when the image forming start signal is output image data is read out from a memory (a RAM, ROM, etc.) or from a memory via registers and the image forming operation starts. However, the situation may arise that the image forming start signal is not synchronized with the detected signal from photodetector 31. As a result, situations as shown in FIGS. 2a and 2b arise in an image forming operation, and during a series of image forming operations. Certain kinds of vertical regist shifts may occur based on the above lack of synchronization between the image forming start signal and the detected signal from the photodetector 31.
When the situations as shown in FIGS. 2a and 2b exist in an image forming operation, a vertical regist shift may occur on output sheets of paper on which a same image is printed, on lines on a sheet of paper, or on colors on a sheet of paper. When the image forming device prints the same image on plural sheets of paper by plural image forming operations, a vertical regist shift may occur between the same position of each line of each paper. Also, when a color image forming device outputs color images on a single sheet of paper, a vertical regist shift may occur between each color image on the sheet. For example, a vertical regist shift may occur between a cyan color image and a magenta color image. A vertical regist shift may also occur between lines in a sheet of paper on a monochrome image forming device.
A normal operation and an operation which results in the vertical resist shift are explained in further detail with reference to FIGS. 2a and 2b which show two cases in the operation of the system of FIG. 1. FIGS. 2a and 2b disclose a range of a vertical regist shift based upon a relation between the image forming start signal and the synchronous signal for the scanning direction (i.e., the detected signal from the photodetector 31).
In the operation of the system of FIG. 1 shown in the second case of FIG. 2b, the synchronous signal for the scanning direction (as detected by photoconductor 31) is generated just after the image forming start signal for the subscanning direction (which starts rotation of the photoconductive drum 14). As a result, in this operation of FIG. 2b the first and second lines of information are immediately written at their appropriate positions.
In this operation of the system of FIG. 1 the writing on the photoconductive drum 14 is determined to start after a predetermined number of synchronous signals are detected (i.e., when a designated synchronous signal is detected) after the image forming start signal for the sub-scanning direction is detected. That is, in the operation of the system shown in FIG. 1 the photodetector 31 detects a signal to start writing onto the photoconductive drum 14, and thereby the photodetector 31 provides an image forming start signal for the scanning direction. Further, the system as shown in FIG. 1 is designed so that a predetermined number of synchronous signals for the scanning direction are generated prior to writing of the first and second lines of information onto the photoconductive drum 14. In the operation shown in FIG. 2b such a predetermined number of synchronous signals prior to the designated synchronous signal for writing are not shown.
In this first case of FIG. 2a a synchronous signal (to initiate writing by the laser beams output of light sources 25 and 26 onto the photoconductive drum 14) for the scanning direction occurs just prior to the image forming start signal for the sub-scanning direction (which starts rotation of the photoconductive drum 14). As a result, the designated synchronous signal for the scanning direction which is detected after the image forming start signal is a significant time tl after the image forming start signal. In this situation, the first and second lines are not written onto the photoconductive drum 14 until approximately one time cycle later than desired. This problem is significant in a multiple beam scanning system because each time cycle for writing corresponds to a shift of two lines if two light sources 25, 26 are being used, or a shift of three lines if three light sources are being used, etc.
One system for addressing this vertical regist shift operation is disclosed in Japanese Laid-Open Patent Application 08-142412. A control operation executed in this Japanese Laid-Open Patent Application 08-142412 is described in FIGS. 3a and 3b of the present specification.
In the system of this further background art a time interval detector for detecting a time from an image forming start signal for a sub-scanning direction and a synchronous signal for the scanning direction is measured, and a light source selector is provided to select between activating the light sources 25 and 26 based on this detected time interval.
More particularly, in this background art in a second case of FIG. 3b if a certain time interval t2 occurs between the image forming start signal for the sub-scanning direction and the synchronous signal for the scanning direction (which time interval t2 is detected by a time interval detector), a first line is written by only one of the light sources, and then for subsequent lines both of the light sources operate to write lines simultaneously. In the example shown in FIG. 3b, with the time interval t2 the first line is written onto the photoconductive drum 14 by the light source 26, and then the second and third lines are simultaneously written by both the light sources 25 and 26 after a second synchronous signal is detected, and so on.
This operation disclosed in Japanese Laid-Open Patent Application No. 08-142412 provides a benefit that a vertical regist shift between lines, colors or pages can be decreased to one line. However, this reduction in the vertical regist shift may still be unacceptable in a multiple beam writing system, and particularly if the multiple beam writing system of FIG. 1 is utilized in a color copier where different colors must be precisely superimposed upon one another. The contents of this reference, Japanese Laid-Open Patent Application No. 08-142,412, are incorporated herein by reference.